1. Field of the Invention
The present invention relates to Earth-orbiting spacecraft or satellite control systems and more particularly to a ground-based control system which interfaces with a satellite's ground station to automatically continue attitude control of a spacecraft in the event of the failure of the momentum wheel or reaction wheel system.
2. Prior Art Problem to be Solved
Various systems are known for the control of Earth-orbiting spacecraft or satellites involving the use of hardware and software for executing complex control algorithms that utilize on-board attitude control equipment such as momentum wheels, gyros, magnetic torquers, and thrusters to maintain and alter the satellite's orientation and orbit. However, problems are posed with such systems when there are failures in the proper operation of an on-board control device. In such event, the equipment and algorithms must be quickly adjusted to deal with the changed situation and maintain the satellite in its desired attitude and orbit. As the on-board control equipment and fuel are self-contained, the required adjustments should be efficient in execution.
One example of a system of this type is the ground loop automatic control system (GLACS) of Telestar of Canada. Pertinent features of the GLACS syst em involve using a combination of on-board roll/pitch sensing and ground-based yaw sensing, and ground-commanded magnetic torquer actuations/thruster firings to perform day-to-day attitude control. In Yaw sensing the incoming signal is aligned 45.degree. to the nominal polarization and t he detector uses two probe s positioned 90.degree. apart to sense two components that are amplified by the system separately, and which components are then compared to produce the Yaw signal. Two basic modes of operation a re provided, namely: normal mode, for day-to-day operations, and acquisition mode, to deal with higher rate situations (post stationkeeping or attitude re-acquisition). The features of these modes are listed in the following Tables I and II.
TABLE I ______________________________________ Normal Mode POSITION RATE ACTUATOR ______________________________________ Yaw APK & EDM RF Derived from Roll torquer yaw sensors position coil with thruster backup Roll ESA/ASP 32 Derived from Yaw torquer coil sample roll position with thruster error telemetry backup Pitch ESA/ASP 2 Derived from Subset of sample pitch position thrusters, 6, 8N, error telemetry 8S, 9N, 9S, 11 ______________________________________
TABLE II ______________________________________ Acquisition Mode POSITION RATE ACTUATOR ______________________________________ Yaw APK & EDM RF Derived from North face yaw sensors integrated thrusters, yaw RMA east/west face angle thruster backup Roll ESA/ASP 32 Derived from North face sample roll integrated thrusters, error telemetry RMA roll east/west face angle or thruster backup roll rate Pitch ESA/ASP 2 Derived from Subset of sample pitch position or thrusters 6, 8N, error telemetry pitch RMA 8S, 9N, 9S, 11 rate ______________________________________
Also, stationkeeping maneuvers are performed by existing on-board processes.
The GLACS system is made feasible by the availability of small thrusters on the spacecraft with which it has been implemented. Many other spacecraft that use larger thrusters and consequently produce larger torques would make the implementation of the GLACS system thereon significantly less fuel efficient. In addition, the GLACS system uses a means of yaw sensing as described above which in many other spacecraft systems would be readily available or easily available. Thus, the GLACS system has limited usefulness for spacecraft in general.
3. Objects
It is accordingly an object of the present invention to provide a more generally applicable system that enables the continued attitude control and simultaneous partial orbit control of a spacecraft in the event of momentum wheel failure with the novel and efficient use of thrusters, magnetic torquers and on-board gyroscopes.